Cuomo has faced criticism from the state's Working Families Party, and the labor-backed party hasn't ruled out running its own candidate because of concerns about Cuomo's policies.

If a Working Families Party candidate were to run, the candidate would immediately pick up 24 percent of the vote, the Siena Poll said, the same as Astorino in a three-way matchup.

"While Cuomo continues to hold a dominant position in a head-to-head matchup against Astorino, his lead is cut in half when a WFP candidate perceived to be more liberal or progressive than Cuomo is added to the mix," said Siena College pollster Steven Greenberg said in a statement.

In a head-to-head race, Cuomo led Astorino 58 percent to 28 percent, down slightly from last month's Siena poll. But with a three-candidate race, Cuomo's lead would drop to a 15 percentage-point edge over Astorino: with 39 percent for Cuomo, and 24 percent each for Astorino and the unnamed Working Families Party candidate.

Support for a Working Families Party candidate was strongest in New York City. Cuomo had the party's line in 2010, but only after it agreed to a number of reforms Cuomo wanted to enact if he was elected.

The Working Families Party is holding its convention May 31 in Albany, and it said in an email to supporters earlier this month, "We'll have a serious decision to make as we endorse a candidate for governor and other statewide elections."

The email ripped Cuomo and the Legislature for not passing an expansive public-financing system for campaigns, as well as providing tax breaks to businesses.

Cuomo's favorability rating was essentially unchanged at 57 percent positive, and his job performance was the same, at 45 percent positive.

Cuomo has sparred with unions since he took office in 2011, and he's viewed as more of a moderate than a liberal, the poll showed. Among all voters, 46 percent described Cuomo as moderate, compared to 35 percent liberal and 11 percent conservative.

"Cuomo's dilemma with a challenge from the left can be at least partly explained by the fact that Democrats see Cuomo as not liberal enough. More than twice as many Democrats describe Cuomo as moderate rather than liberal," Greenberg said.

Albany bureau chief:
Joseph Spector is Gannett's Albany Bureau chief and has covered New York politics and government since 2002. He was the political reporter for the Democrat and Chronicle in Rochester and in 2007 joined Gannett’s Albany Bureau, covering state government for seven news organizations&nbsp;and USA Today.